1. Field of the Invention
The invention relates to disk drive systems which have a controller which employs a look ahead operating system for processing a look ahead command set and a tag queuing operating system for processing a tag queuing command set issued by a host processor.
2. Description of the Related Art
A disk drive system will perform best using a control operating system that has been optimized to the command set that is used by the host processor for communicating with the disk drive system.
A first command set used by host processors is the look ahead command set which requires the host processor to await the completion of a command issued to the disk drive system before issuing the next command and for the disk drive system to perform multiple threaded sequential operations. In this case the disk drive system will perform best by using a look ahead operating system that is optimized to perform n-segment look ahead operations that anticipate what the next command from the host computer will be and will pre-fetch data associated with the next anticipated command to avoid latencies. The use of the optimized look ahead operating system in communicating with the host processor will increase the computing system's performance.
A second command set used by host processors is the tag queuing command set where the host computer issuing multiple commands at one time to the disk drive system and the disk drive system receives and store the command for subsequent processing of the commands by the disk drive system. When the tag queuing command set is used, it is no longer necessary for the disk drive system to anticipate what command the host processor will next issue because with more than one command stored in the queue in the disk drive system, the disk drive system will know exactly what the host computer wants to do next. Therefore, the disk drive system will best operate using an optimized tag queue operating system to take advantage in the processing of the multiple outstanding commands so as to boost the performance of the system and to not waste time doing unnecessary look ahead operations.
Disk drive system manufacturers may produce an optimized disk drive system for use with host processors employing the look ahead command set and an optimized disk drive system for use with host processors employing the tag queuing command set. This approach has the undesirable effect of requiring that two models of the same disk drive system must be marketed where the only difference is the operating system used in communicating with the host processor. Therefore disk drive system manufacturers have endeavored to provide a disk drive system which is capable of responding to both the look ahead command set and the tag queuing command set. However, in attempting to provide a single operating system so as to make the disk drive system transparent to the command set being used by the host computer, the optimization of the operating system for both of the command sets had to be compromised. This compromise results in the decrease in efficiency of the system from the system efficiency that could have been obtained if the optimum operating systems could have been used for the command sets.